Isolation and purification of single walled carbon nanotube structures

ABSTRACT

Disclosed are methods for isolating and purifying single wall carbon nanotubes from contaminant matrix material, methods for forming arrays of substantially aligned nanotubes, and products and apparatus comprising a plurality of nanotube structures.

CROSS-REFERENCE TO RELATED APPLICATION

This applications claims the benefit of U.S. patent application Ser. No.11/479,672 entitled “Isolation and Purification of Single Walled CarbonNanotube Structures,” filed Jun. 30, 2006 which is a continuation ofU.S. Provisional Patent Application No. 60/303,816 entitled “Isolationand Purification of Single Walled Carbon Nanotube Structures,” filedJul. 10, 2001, which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates to methods for isolating and purifyingsingle-walled carbon nanotubes from contamination materials, such ascarbon and metal catalyst particles, present in the unpurified materialfollowing production of the single-walled carbon nanotube structures.Specifically, the present invention relates to utilizing solution ofsuitable dispersal agents to isolate and purify individual single-walledcarbon nanotube structures from a raw material including bundles ofnanotube structures.

DESCRIPTION OF THE RELATED ART

There has been significant interest in the chemical and physicalproperties of carbon nanotube structures since their discovery in 1991,due to the vast number of potential uses of such structures,particularly in the field of nanotechnology, composite materials,electronics and biology. Accordingly, there has been an increase indemand in recent years for carbon nanotube structures for research andapplication purposes, resulting in a desire to produce in an efficientmanner single-walled carbon nanotube (hereinafter referred to as“SWCNT”) structures that are free of impurities and easily separable fortheir proper characterization.

The three most common manufacturing methods developed for the productionof SWCNT structures are high pressure carbon monoxide (“hipCO”)processes, pulsed laser vaporization (“PLV”) processes and arc discharge(“ARC”) processes. Each of these processes produce SWCNT structures bydepositing free carbon atoms onto a surface at high temperature and/orpressure in the presence of metal catalyst particles. The raw materialformed by these processes includes SWCNT structures formed as bundles oftubes embedded in a matrix of contamination material typically composedof amorphous carbon (i.e., graphene sheets of carbon atoms not formingSWCNT structures), metal catalyst particles, organic impurities andvarious fullerenes depending on the type of process utilized. Thebundles of nanotubes that are formed by these manufacturing methods areextremely difficult to separate into individual nanotube filaments.

In order to fully characterize the physical and chemical properties ofthe SWCNT structures formed (e.g., nanotube length, chemicalmodification and surface adhesion), the contaminating matrix surroundingeach structure must be removed and the bundles of tubes separated anddispersed such that each SWCNT structure may be individually analyzed.By maintaining an appropriate dispersal of individual SWCNT structures,characterization of the nanotubes formed may be accomplished in amechanistic manner. For example, it is desirable to be able to easilyanalyze and characterize dispersed SWCNT structures (e.g., determinechange in nanotube length, tensile strength or incorporation of definedatoms into the carbon matrix of the SWCNT structure) based upon amodification of one or more steps of a manufacturing method.

It is further highly desirable to produce individual and discrete SWCNTstructures in a form rendering the structures easily manipulated for usein the previously noted fields. At best, existing methodologies capableof physically manipulating discrete material components require elementsthat are measured on micron-level dimensions rather than the nanometerlevel dimensions of conventional, partially dispersed and purified SWCNTstructures. However, biological systems routinely manipulate withprecise spatially oriented discrete elements (e.g., proteins) havingphysical dimensions on the order less than SWCNT structures. Thus, ifSWCNT structures could be biologically derived so that biological“tools,” such as immunoglobulins or epitope-specific binding proteins,could be utilized to specifically recognize and physically manipulatethe structures, the possibility of accurately spatially orienting SWCNTstructures becomes feasible. In order for this approach to be realized,the individual SWCNT structures should be substantially separated fromthe raw material for the optimal functioning of biological compoundsduring both the biological SWCNT derivitization and the manipulationprocesses. In other words, in order to effectively manipulate thestructures, it is highly desired that the SWCNT structures be producedas individual, freely dispersed structures in an aqueous buffer systemthat exhibits a nearly neutral pH at ambient temperatures.

Current methods for purifying and isolating SWCNT structures fromcontaminating matrix surrounding the structures employ a variety ofphysical and chemical treatments. These treatments include: the use ofhigh temperature acid reflux of raw material, which attempts tochemically degrade contaminating metal catalyst particles and amorphouscarbon, the use of magnetic separation techniques to remove metalparticles, the use of differential centrifugation for separating theSWCNT structures from the contaminating material, the use of physicalsizing meshes (i.e., size exclusion columns) to remove contaminatingmaterial from the SWCNT structures, and the use of sonication tophysically disrupt the raw material into its components. Additionally,techniques have been developed to partially disperse SWCNT structures inorganic solvents in an attempt to purify and isolate the structures.

All of the currently available methods are limited for a number ofreasons. Initially, it is noted that current purification methodsprovide a poor yield of purified SWCNT structures from raw material. Afinal SWCNT product obtained from any of these methods will alsotypically contain significant amounts of contaminating matrix material,with the purified SWCNT structures obtained existing as ropes or bundlesof nanotubes thereby making it difficult to analyze and characterize thefinal SWCNT structures that are obtained. These methods furthertypically yield purified SWCNT structures of relatively short lengths(e.g., 150-250 nm) due to the prolonged chemical or physical processingrequired which causes damage to the nanotubes. Additionally, a number ofisolation techniques currently utilized require organic solvents orother noxious compounds which create environmental conditions unsuitablefor biological derivitization of SWCNT structures. Organic solventscurrently utilized are capable of solubilizing SWCNT structures inbundles and not individual, discrete tubes. Furthermore, presentisolation techniques require prolonged periods of ultra-speedcentrifugation (e.g., above 100,000×g) in order to harvest nanotubestructures from solvents or other noxious compounds used to removecontaminating matrix material from the nanotubes.

A further problem occurs during characterization analysis of the SWCNTstructures. One form of analyzing SWCNT structures is through the use oftransmission electron microscopy (hereinafter referred to as “TEM”), amagnification process which allows one to visualize the SWCNTstructures. TEM analysis typically uses electron microscopy supportfilms such as specialized grids made of polymeric materials, e.g.,modified polyvinyl acetal resins manufactured by Chisso Corporation,Osaka, Japan, under the trademarks FORMVAR® and VINYLEC® to capturenanotube material contained in solution in a manner analogous to afilter. As liquid containing the SWCNT structures passes through aFORMVAR® grid, a layer of SWCNT structures is captured and, even ifdispersed (e.g., in an organic solvent), re-associates into ropes orbundles of nanotubes. A TEM image illustrated in FIGS. 1 a and 1 b showsan example of the condition of SWCNT structures after purification andpartial dispersion in a solution of methanol, as taught in the priorart. The SWCNT structures of FIG. 1 a form tangled bundles upondeposition on a FORMVAR® grid. The image in FIG. 1 b, which is amagnification of FIG. 1 a, further shows the presence of metal catalystimpurities embedded within the nanotube rope structures (e.g., indicatedby the arrows) which shows the inability of conventional purificationmethods to substantially remove contaminants from the SWCNT material.

Presently, the overwhelming problem for industrial and academiclaboratories engaged in the use of carbon nanotubes for research as wellas other applications is the limited source of discrete, completelyseparated SWCNT structures. Investigations into the vast potential ofuses for SWCNT structures are being hampered by the limited supply ofwell characterized SWCNT structures free of significant amounts ofcontaminants like amorphous carbon and metal catalyst particles.

Accordingly, there presently exists a need for harvesting high yields ofpurified SWCNT structures from the raw material of a carbon nanotubeproduction process in a fast and efficient manner to meet the demand forsuch structures. Additionally, it is desirable to provide SWCNTstructures as discrete and individual structures (i.e., not bundledtogether), having suitable lengths and well characterized for biologicalderivitization and easy manipulation.

SUMMARY OF THE INVENTION

Therefore, in light of the above, and for other reasons that will becomeapparent when the invention is fully described, an object of the presentinvention is to provide a rapid and an effective method of isolating andpurifying SWCNT structures disposed within a raw material containingcontaminants to obtain a high product yield of quality SWCNT structureshaving appropriate lengths suitable for different applications.

Another object of the present invention is to provide a method ofdispersing isolated and purified SWCNT structures in solution from theraw material so as to yield discrete and separated nanotube structuressuitable for different applications.

A further object of the present invention is to provide a method ofdispersing isolated and purified SWCNT structures in a suitable solutionto render the structures suitable for biological derivitizationprocedures to effect easy manipulation and characterization of the SWCNTstructures.

The aforesaid objects are achieved in the present invention, alone andin combination, by providing a method of dispersing a matrix of rawmaterial including SWCNT structures and contaminants in an aqueoussolution containing a suitable dispersal agent to separate theindividual SWCNT structures from the matrix, thus purifying anddispersing the structures within the solution. In solution, thedispersal agent surrounds and coats the individual SWCNT structures,allowing the structures to maintain their separation rather thanbundling together upon separation of the structures from the solution.Suitable dispersal agents useful in practicing the present invention aretypically reagents exhibiting the ability to interact with hydrophobiccompounds while conferring water solubility. Exemplary dispersal agentsthat can be used in the present invention include, but are not limitedto, synthetic and natural detergents, deoxycholates, cyclodextrins,chaotropic salts and ion pairing agents.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of specific embodiments thereof,particularly when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a transmission electron microscopy (“TEM”) image of rawmaterial resulting from a pulsed laser vaporization process after it ispartially purified using a conventional purification process employingorganic solvents. As shown in the TEM image, the raw material containsSWCNT structures and numerous metal catalyst particles and amorphouscarbon material. (Scale bar=200 nm).

FIG. 1 b is an enlargement of the TEM image of FIG. 1 a. The arrowsindicate metal catalyst particles which are present among a plurality ofnanotube structures. (Scale bar=10 nm).

FIG. 2 a is a graphical representation of percent transmission (“% T”)values for aqueous solutions containing three synthetic detergentshaving varying surfactant strengths.

FIG. 2 b is a graphical representation of percent transmission (“% T”)versus time for the aqueous solutions of FIG. 2 a, wherein the solutionshave undergone evaporation.

FIG. 2 c is a graphical representation of percent transmission (“% T”)versus time for aqueous solutions of FIG. 2 a, wherein the solutionshave undergone no evaporation.

FIG. 3 is a graphical representation of percent transmission (“% T”)versus time for aqueous solutions containing taurocholic acid andmethyl-β-cyclodextrin.

FIG. 4 is a TEM image of “raft-like” SWCNT structures filtered onto aFORMVAR® grid after being dispersed in an aqueous methyl-β-cyclodextrinsolution.

FIG. 5 is a graphical representation of % T values for fractionscollected during fractionation of a methyl-β-cyclodextrin solution ofdispersed SWCNT structures in a 5000 MW size exclusion column.

FIGS. 6 a-6 d depict atomic force microscopy (hereinafter referred to as“AFM”) images of SWCNT structures resulting from fractionation of amethyl-β-cyclodextrin solution of dispersed SWCNT structures afterdeposition on a glass coverslip and air drying at 37° C. for 1 hour.

FIGS. 7 a-7 d depict AFM images of SWCNT structures captured within alayer of PEG coated glass coverslip.

FIGS. 8 a-8 d depict AFM images of a glass coverslip coated withpolyethylene glycol and containing SWCNT “raft-like” structures formedafter controlled evaporation of water from a methyl-β-cyclodextrinsolution of dispersed SWCNT structures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for purifying and isolationSWCNT structures from raw material by dispersing the structures in anaqueous solution with a biologically active dispersal agent. Thebiologically active dispersal agent effects a separation of the SWCNTstructures from contaminated material such that the purified SWCNTstructures exist as a dispersion of individual and discrete SWCNTstructures in solution. As used herein, the term “raw material” refersto material formed by any process for producing single-walled carbonnanotubes, including, without limitation, the three processes describedabove. The raw material typically contains SWCNT structures embedded ina matrix of contaminated material. The terms “contaminated material” and“contaminants,” as used herein, refer to any impurities or othernon-SWCNT components in the raw material including, without limitation,amorphous carbon and metal catalyst particles.

As previously noted, the current methods employed for purifying andharvesting SWCNT structures have met with limited success due in part tothe traditional view of SWCNT structures as chemical compounds. In adeparture from the traditional view, SWCNT structures are consideredhere as being similar to biologically derived structures. Some notedproperties of SWCNT structures are as follows (not all of which must bepresent): (i) they are typically insoluble in water; (ii) they typicallyself associate as bundles or ropes; (iii) they are made exclusively ofcarbon; and (iv) each end of a carbon nanotube will typically exhibitdifferent physiochemical properties. The physical properties of carbonnanotubes are in fact very similar to lipids, which are a class ofbiological compounds insoluble in water but capable of being solubilizedin aqueous solutions including suitable lipid dispersing reagents. Assuch, the inventors recognized that SWCNT structures are readilydispersable within an aqueous solution containing a reagent typicallysuitable for dispersing proteins or lipids in aqueous solutions.

Reagents considered effective in suitably dispersing SWCNT structures inaqueous solution are referred to as dispersal agents. A dispersal agentcan be any suitable reagent that is effective in substantiallysolubilizing and dispersing SWCNT structures in an aqueous solution byincreasing the interaction at the surface interface between eachnanotube structure and water molecules in solution. The dispersal agentis typically added to an aqueous solution in an effective amount tosubstantially purify and to disperse SWCNT structures in solution. Theeffective amount of dispersal agent will vary based upon the type ofdispersal agent utilized in a particular application.

The dispersal agents are typically synthetic or naturally occurringdetergents or any other composition capable of encapsulating andsuitably solubilizing hydrophobic compounds in aqueous solutions.Exemplary dispersal agents include, without limitation, synthetic ornaturally occurring detergents having nonionic or anionic surfactantactivities such as: alkylaryl polyether alcohols, e.g.octylphenol-polyethylene glycol ether, commonly sold under thetradename, Triton® X-100 (hereinafter referred to as “TX-100”)(Sigma-Aldrich, St. Louis, Mo.); phenylated polyethoxy ethanols, e.g.(nonylphenoxy) polyethoxy ethanol commonly sold as Nonidet P-40/NP-40(Vysis, Inc., Downers Grove, Ill.)(hereinafter “NP-40”); polyoxyethylenesorbitol esters, e.g., TWEEN®, manufactured by ICI Americas, Inc.,Bridgewater, N.J. and EMASOL™, manufactured by Kao Specialties AmericasLLC, High Point, N.C.; sodium dodecyl sulfate (hereinafter “SDS”); andammonium bromides and chlorides (e.g., cetyltrimethylammonium bromide,tetradecylammonium bromide and dodecylpyrimidium chloride). Otherdispersal agents include, without limitation, naturally occurringemulsifying agents such as deoxycholates and deoxycholate-typedetergents (e.g., taurocholic acid) and cyclodextrins (e.g., α-, β- orγ-cyclodextrin), chaotropic salts such as urea and guanidine, and ionparing agents such as sulfonic acids (e.g., 1-heptane-sulfonic acid and1-octane-sulfonic acid).

Naturally occurring emulsifying agents such as taurocholic acid andcyclodextrins are highly effective in solubilizing and dispersing SWCNTstructures and in facilitating biological derivitization of the purifiedand isolated SWCNT structures. In particular, cyclodextrins have a threedimensional doughnut shaped orientation with a “torsional” structurecomposed of glucopyranose units. The “torsional” structure of acyclodextrin molecule allows it to attract and encapsulate a SWCNTstructure within its central hydrophobic region, even when physicallyaltered from a round “doughnut” shape to a twisted “doughnut” shape,while maintaining an outer hydrophilic surface rendering the moleculesoluble in aqueous solutions. The solubility of cyclodextrins in watermay also be increased nearly tenfold by substitution of, for example,methyl or hydroxypropyl groups on the cyclodextrin molecule. Greatersolubility of the cyclodextrin in water translates to a greaterdispersion and isolation of individual SWCNT structures in solution. Twoexemplary cyclodextrin derivatives that are highly effective indispersing SWCNT structures in solution are methyl-β-cyclodextrin(hereinafter referred to as “MβC”) and 2-hydroxypropyl-β-cyclodextrin(hereinafter referred to as “2-HP-β-C”). However, it is noted that anycyclodextrin (e.g., α, β or γ), or any suitable derivative thereof, maybe utilized in accordance with the present invention. Further,cyclodextrins are useful for biological derivitization of SWCNTstructures which have been isolated in solution. Taurocholic acid(“TA”), which is exemplary of a suitable deoxycholate-type detergentcapable of substantially dispersing SWCNT structures in solution, isproduced naturally in mammalian liver tissue. It is also highlyeffective in facilitating biological derivitization of purified andisolated SWCNT structures because, like the cyclodextrins, TA has amolecular shape that allows a large surface area of SWCNT structures tobe coated per molecule of TA. Typically, cyclodextrins and deoxycholatesmay be utilized to suitably disperse SWCNT structures according to thepresent invention in concentrations ranging from about 5 mg/ml to about500 mg/ml of aqueous solution, and in some embodiments, in aconcentration of about 50 mg/ml.

Synthetic detergents suitable for use as dispersal agents here willtypically have high surfactant activity and be utilized in amounts ofabout 50 to about 95% of their critical micelle concentration(hereinafter “CMC”) values. These high surfactant detergents are capableof overcoming hydrophobic forces at the SWCNT surface/aqueous solutioninterface by coating the SWCNT structures to establish suitablesolubility of the SWCNT structures in solution. As appreciated by thoseof skill in the art, the surfactant properties of a synthetic detergentmay be characterized in terms of a hydrophilic-lipophilic balance(hereinafter “HLB”), which provides a measurement of the amount ofhydrophilic groups to hydrophobic groups present in a detergentmolecule. In particular, synthetic detergents that are suitable for useas dispersal agents here have an HLB value between about 7 and about13.2. Limiting the concentration of the synthetic detergent to asuitable level below its CMC will also ensure adequate dispersion of theSWCNT structures without the formation of floccular material.Additionally, chaotropic salts (e.g., urea and guanidine) are typicallyutilized as dispersal agents in concentrations ranging from about 6M toabout 9M in solution (wherein “M” refers to molarity), whereas ionpairing agents are typically utilized as dispersal agents inconcentrations ranging from about 1 mM to about 100 mM in solution.

While selection of a suitable dispersal agent as well as a suitableconcentration is important for achieving a desirable dispersion of SWCNTstructures in aqueous solution, other factors may also enhance thedispersing effect of the dispersal agent. Exemplary factors that affectdispersion of SWCNT structures in aqueous solutions include, withoutlimitation, the pH of the solution, cation concentration (e.g., sodium,potassium and magnesium) in solution, and other conditions such asoperating temperature and pressure. Dispersal agent molecules typicallysurround individual SWCNT structures and separate those structures frombundles of tubes in solution.

The raw material containing SWCNT structures typically is added to anaqueous solution containing the dispersal agent and appropriately mixed(e.g., by mechanical agitation or blending) to ensure adequateinteraction and coating of dispersal agent molecules with SWCNTstructures. While the amount of SWCNT material that may be added to anaqueous dispersal agent solution to obtain an effective dispersion ofSWCNT structures typically depends upon factors such as the specificdispersal agent utilized and its concentration in solution, effectivedispersions have been achieved utilizing concentrations as high as about1 mg/ml of SWCNT structures in aqueous dispersal agent solution. Uponadequate mixing, the dispersed nanotube solution, i.e., the solutioncontaining the dispersed SWCNT structures, may be filtered with anappropriately sized filter (e.g., about 0.05 to about 0.2 μm filtration)to remove any insoluble material (e.g., matrix contaminants) remainingin solution. Typically, a 0.2 μm filter is utilized to ensure adequateremoval of contaminants while preventing caking of the filter and lossof dispersed SWCNT structures. However, smaller pore size filters mayalso be utilized to ensure more efficient removal of contaminants. Insituations where a smaller pore size filter is implemented, any SWCNTstructures that may have become trapped in the filter cake may berecovered by resuspension of the cake in dispersal agent solution andrepeating filtration steps as necessary to obtain a desirable yield.

Additional processing steps, such as centrifugation or other separationtechniques, may also be utilized to remove insoluble material and excessdispersal agent from solution after the SWCNT structures have beensuitably dispersed therein. Specifically, the SWCNT structures may bewashed to remove excess dispersal agent by subjecting the solution tocentrifugation at speeds ranging from about 100×g to about 10,000×g tosediment SWCNT structures. The SWCNT structures may then be removed fromsolution and re-dispersed in distilled water. The washing process may berepeated any desired number of times to ensure adequate removal ofexcess dispersal agent. The SWCNT structures may also be separated fromexcess dispersal agent and other contaminants in solution via dialysisor the use of an appropriate size exclusion column (e.g., a 5000 MW sizeexclusion column). The resultant solution, which contains substantiallyisolated and purified SWCNT structures coated with dispersal agent, ishighly useful in a variety of applications, particularly nanotechnologyresearch.

An additional feature relates to the removal of SWCNT structures fromsolution while preventing the structures from re-bundling together.Specifically, the isolated and purified SWCNT structures in dispersalagent solution may be deposited on a suitable substrate in theirindividual and discrete form. For example, SWCNT structures coated withdispersal agent may be deposited on a FORMVAR® grid for TEM analysis.The deposited SWCNT structures typically form substantiallylongitudinally aligned and substantially parallel “raft-like” or tapestructures that are free of any contaminants. As used herein,“raft-like” or “tape” refers to arrays of nanotubes arranged in variousgeometrically ordered configurations, including configurations whereindividual nanotubes are placed generally parallel with respect to eachother to form structures of monolayer or multi-layer thicknesses. Thealignment of SWCNT structures into substantially parallel “rafts” occursdue to repulsive forces induced by the dispersal agent coated surfacesof the structures. The highly ordered and separated alignment ofindividual nanotubes facilitates easy characterization and manipulationof the SWCNT structures. As previously noted, current methods forisolating nanotube structures on a surface such as a FORMVAR® grid haveled to a tangled mess of nanotubes having contaminated material embeddedtherein, as clearly indicated in FIGS. 1 a and 1 b. Thus, the isolationand purification methods of the present invention result in a novelformation of “raft-like” SWCNT structures. Another method for forming“raft-like” SWCNT structures is to immobilize the structures on apoly-hydroxylated surface. For example, dispersal agent coated SWCNTstructures may be deposited on a surface coated with polyethyleneglycol, e.g., a low molecular weight polyethylene glycol (“PEG”) such asCarboWax® (Dow Chemical Co., South Charleston, W. Va.). Subsequentanalysis reveals that the SWCNT structures remain in isolated form.

Deposition and capture of dispersal agent coated SWCNT structures on asurface such as those previously described provides a permanent recordof the structures in isolated form, which is important for conductingcharacterization studies of the structures utilizing AFM. AFM analysisprovides a highly accurate determination of the dimensions of singleSWCNT structures, including overall length and diameter. AFM furtherprovides the spatial resolution required to both distinguish individualSWCNT structures from nanotube bundles or ropes, and allow individualSWCNT structures to be imaged along their full lengths. Utilizing AFManalysis, the dispersal agent coated SWCNT structures separated from rawmaterial according to the present invention can be easily visualized intheir isolated and purified form having lengths on the order of about 10to about 15 μm. It is noted that previous reported SWCNT lengthsutilizing other known isolation and purification techniques are on theorder of only about 150 to about 250 nm. Additionally, AFM analysisreveals surface-deposited SWCNT structures coated with a dispersal agentyield “raft-like” formations in which both single layers and multiplelayers, up to 4 layers thick, form on the substrate surface.

The following examples disclose specific methods for isolating andpurifying SWCNT structures from raw material containing contaminants.Specifically, NP-40, TA and a cyclodextrin derivative are utilized toshow the effect of each in dispersing SWCNT structures in aqueoussolution. The raw material containing SWCNT structures for each examplewas obtained utilizing a PLV process. However, it is noted that theSWCNT structures may be isolated and purified utilizing raw materialprovided via other processes and still be within the scope of thepresent invention. It is further noted that the examples are forillustrative purposes only and in no way limit the methods and range ofdispersal agents contemplated by the present invention.

EXAMPLE 1

Raw material containing bundles SWCNT structures was mixed into threesynthetic detergent solutions known for solubilizing proteins and lipidsin aqueous solutions. The three synthetic detergents utilized wereNP-40, SDS and TX-100. These detergents were selected due to theirdiffering physical properties and to demonstrate how the surfactantactivity of the detergent affects the dispersion of SWCNT structures insolution. SDS is a strong anionic detergent that solubilizes compoundsin water by virtue of coating the compounds with a layer ofnegatively-charged, water soluble detergent molecules. In contrast, bothTX-100 and NP-40 are non-ionic detergents that function via hydrophobicinteractions with the surface of a compound. thereby forming a watersoluble layer of detergent molecules around the water insolublecompound. The surfactant properties (i.e., ability to decrease surfacetension between aqueous and non-aqueous phases) for NP-40 are muchgreater than SDS and TX-100. Reported HLB values for each of thesedetergents are as follows (e.g., see Kagawa, Biochim. Biophys. Acta 265:297-338 (1972) and Helenius et al, Biochim. Biophys. Acta 415: 29-79(1975)):

Detergent HLB SDS 40 TX-100 13.5 NP-40 13.1

Three aqueous solutions were each prepared as follows. A 1 mg (total dryweight) amount of raw material was solubilized in 1 ml of doubleglass-distilled, deionized water (hereinafter “ddH₂O”) containing one ofthe detergents (e.g., SDS, TX-100 or NP-40) at 50% of its respective CMCvalue. Each solution was subsequently vortexed for 30 minutes at roomtemperature. The resultant dispersions were passed through a 0.2 μmcellulose acetate filter to remove any particulate matter. Conventionalspectroscopy methods were employed to measure the percent transmission(“% T”) of each solution at a wavelength of 450 nm (path length of 3mm).

The % T value of each the solutions was measured to provide anindication of solution color and to comparatively determine the abilityof each detergent to effectively disperse SWCNT structures withinsolution. Specifically, % T values are inversely proportional to thedegree of color in solution. If SWCNT structures are bundled together ina particular solution, floccular material forms which in effect removeSWCNT structures from solution by sedimentation, and thus decreases thecolor and increases the % T value over time. Alternatively, SWCNTstructures remaining dispersed in solution would increase the solutioncolor and thus render a lower % T value. Therefore, a lower % T valuemeasured in the filtrate would indicate a higher level of dispersion ofSWCNT material in solution.

The plots illustrated in FIGS. 2 a-2 c provide % T data for solutionscontaining SDS, TX-100 and NP-40, respectively, with and without SWCNTstructures. T he unshaded bar portions in FIG. 2 a represent % T valuesmeasured for each detergent solution absent any raw material. The % Tvalue for the shaded bar portions represent % T values measured for eachdetergent solution containing SWCNT structures at a time shortly afterfiltration of the solution. The shaded bar data of FIG. 2 a clearlyindicates that NP-40, which has the greatest surfactant properties, hasa much lover % T value than both SDS and TX-100 and thus provides asubstantially more effective dispersion of SWCNT structures in aqueoussolution.

To illustrate the effect of detergent concentration on SWCNT dispersionin solution, the solutions containing SWCNT structures were allowed toevaporate from an initial volume of 150 μl to a final volume of 50 μlover a period of 16 hours at room temperature. Intermittent % Tmeasurements were taken, and the results are illustrated in FIG. 2 b.The % T values for each solution containing a detergent and SWCNTstructures increased with time (i.e., correlating with a decrease incolor), which coincided with a noticeable appearance of floccularmaterial in the detergent dispersions thus indicating that nanotubeswere beginning to re-associate into larger bundles that were insolublein water. The test results indicate that, as the detergent concentrationincreases above its CMC value, micelle formations occur in solutionresulting in reduced dispersion of the SWCNT structures. Thus, selectionof detergent concentration is very important in maintaining dispersionof the SWCNT structures in solution.

A further test was conducted with solutions prepared in a substantiallysimilar manner as the previous solutions. However, these solutions werestored in sealed vials at room temperature so as to prevent theirevaporation. As indicated by the data depicted in FIG. 2 c, there wasrelatively no change in % T value for each of the different detergentsolutions and no noticeable appearance of floccular material after a 72hour period.

The data of example 1 indicates that a strong surfactant such as NP-40is highly effective in dispersing SWCNT structures in aqueous solutionswhen utilized in an effective amount. Further, NP-40 can maintain asuitable dispersion of the structures in solution for extended periodsof time. Weaker surfactants having HLB values greater than 13.2, such asSDS and TX-100, may provide some dispersion but will not be effective insubstantially isolating and purifying SWCNT structures from rawmaterial.

EXAMPLE 2

Aqueous solutions of each of the TA and MβC were prepared alone and withraw materials as follows. Specifically, each solution was prepared bysolubilizing 1 mg of the raw material in 1 ml of ddH₂O containing 50mg/ml of either TA or MβC at 50 mg/ml. Each resultant solution wasvortexed for 30 minutes at room temperature and then filtered through a0.2 (μm cellulose acetate filter. The % T values were measured for thefiltrates at room temperature in sealed vials for 72 hours and comparedwith aqueous solutions containing only TA and MβC.

The % T values illustrated in FIG. 3 reveals that the SWCNT structuresremained dispersed in the TA and MβC filtrates for the entire 72 hourperiod, as is evident from the relatively constant % T values measuredfor each filtrate over that time period. The data further indicates MβCfiltrates had considerably lower % T values, correlating to a greaterdispersion of SWCNT structures, than the TA filtrate and the NP-40filtrate of FIG. 2 c. No significant increases in % T values wereobserved even after a second round of 0.2 μm filtration of each filtrateafter the 72 hour period. The results provided in FIG. 3 clearlyindicate that both TA and MPβC serve as highly effective dispersalagents, providing substantial dispersion of the SWCNT structures inaqueous solution for extended periods of time.

EXAMPLE 3

SWCNT structures dispersed in the TA and MβC solutions of the previousexample were separated from the impurities in solution bycentrifugation. Specifically, SWCNT structures sedimented out of a 1 mlvolume of either solution having a liquid column height of 2.5 cm at acentrifugation speed of 10,000×g. It is noted that prior SWCNTpurification techniques typically require centrifugation speeds inexcess of 100,000×g to yield any sedimentation of SWCNT structures.

The TA and MβC solutions containing SWCNT structures were also subjectedto TEM analysis, wherein 50 μl of each solution filtrate was depositedonto a FORMVAR® grid and the liquid was drawn through the FORMVAR®membrane by placing a clean absorbent pad beneath the grid (i.e., bycapillary action). As the liquid was drawn through the grid, SWCNTstructures formed on the membrane. Images of SWCNT structures were takenat locations where the structures spanned the holes in the membrane. Anexemplary TEM image of the grid is depicted in FIG. 4. The imagesrevealed highly organized SWCNT structures that were aligned in parallel“raft-like” formation, rather than tangled together in bundles or ropes.The structures were also free of metal catalyst particles or otherimpurities. TEM analysis provides a further indication that the SWCNTstructures are dispersed as single discrete nanotubes coated with eitherTA or MβC in the aqueous solutions in order to form the spatial“raft-like” arrangement on the FORMVAR® grid. Additionally, the TEMimages revealed that the coating of either TA or MβC on the SWCNTstructures promotes repulsion between the individual nanotubes,resulting in spatial separation and parallel “raft-like” formations ofindividual SWCNT structures wherein the least amount of surface areacontact between coated nanotubes is tolerated.

EXAMPLE 4

Aqueous MβC solutions containing dispersed SWCNT structures wereprepared as follows. Two hundred μg of SWCNT containing raw material wassolubilized in a 1 ml solution of ddH₂O containing 50 mg/ml of MβC. Thesolution was physically homogenized in a miniaturized inversion blenderat about 23,000 RPM. The resultant dispersion was subsequently vortexedfor 30 minutes at room temperature followed by 100×g centrifugation for10 minutes to sediment any remaining insoluble material. The resultantsupernatant was then passed through a 5000 MW cut-off gravity-fed sizeexclusion column (10 ml bed volume) in the following manner. One ml ofthe dispersed solution was placed on the top of the column, which hadbeen conditioned with 50 ml of ddH₂O. One ml fractions were thencollected from the base of the column as ddh₂O was added to the top ofthe column. The % T values were measured for each collected fraction. Aplot of the % T values versus fraction collected (fractions aresequentially numbered 1 to 20) is illustrated in FIG. 5. Coloredfractions, as indicated by the decreasing % T values, were indicative ofdispersions in solution. Those colored fractions (i.e., fractionsnumbered 1 through 10 of FIG. 5) were collected and pooled together.This procedure was conducted to remove excess MβC from the SWCNTdispersions. The resultant solution containing the dispersed SWCNTstructures was centrifuged at 10,000×g to sediment SWCNT structures fromsolution. The supernatant was resuspended in distilled water inpreparation for use with the examples described below.

EXAMPLE 5

A SWCNT dispersed solution prepared according to the method of Example 4was continuously washed in order to remove as much MβC as possible priorto AFM analysis. Specifically, the solution was subjected to repeatedcentrifugation followed by removal of the resultant supernatant andresuspension in distilled water. The centrifugation and washing processwas repeated a total of four times to remove any excess MβC from thedispersion. A 25 (μl aliquot of the final washed solution was depositedon a 12 mm glass coverslip and allowed to air dry at 37° C. for onehour. When this surface was analyzed utilizing AFM, imaging revealed thepresence of both discretely separated SWCNT structures about 1.4 nm indiameter and larger ropes or bundles of nanotubes about 6-10 nm indiameter as illustrated by the representative AFM image depicted inFIGS. 6 a-6 d (FIG. 6 a depicts the AFM height profile, FIG. 6 b depictsthe AFM amplitude profile, and FIGS. 6 c and 6 d are magnifications ofFIGS. 6 a and 6 b, respectively). The AFM images indicate that removalof the majority of MβC from solution by repeated washing resulted in there-association of some of the SWCNT structures back into ropes orbundles, while other SWCNT structures remained separated and inisolation. In effect, this example illustrates that dispersal of SWCNTstructures in an aqueous solution will decrease if the dispersal agentis reduced below an effective amount in solution.

EXAMPLE 6

An AFM surface was developed to specifically capture MβC-coated SWCNTstructures in a suitable manner to effect proper characterization of thestructures. Specifically, the surface of a 12 mm round glass coverslipwas coated with a layer of a low molecular weight polyethylene glycol,e.g. PEG-200, available commercially as CarboWax® by Dow Chemical Co.,South Charleston, W. Va.), and 25 μl of an aqueous MβC solutioncontaining dispersed SWCNT structures, prepared according to the methodof Example 4, was deposited on the coverslip, quickly washed to removeexcess MβC and then allowed to air dry at room temperature. When thedried surface was analyzed using AFM imaging, discretely separated SWCNTstructures were observed as being attached to the PEG coated surface asillustrated by the representative AFM image depicted in FIGS. 7 a-7 d(FIG. 7 a depicts the AFM height profile, FIG. 7 b depicts the AFMamplitude profile, and FIGS. 7 c and 7 d are magnifications of FIGS. 7 aand 7 b, respectively). The arrows in FIGS. 7 c and 7 d identifydiscretely separated SWCNT structures, whereas the arrow heads identifyPEG absorbed on the glass substrate. The AFM images further reveal SWCNTstructures from 10- 15 μm in length, i.e., verifying that the dispersalmethods of the present invention yield SWCNT structures of much greaterlengths than the typical 150-250 nm lengths yielded by current isolationand purification techniques. Thus, this example illustrates that SWCNTstructures dispersed in aqueous dispersal agent solutions may be fullycharacterized by capturing the structures on poly-hydroxylated surfacessuch as a PEG coated glass coverslip.

EXAMPLE 7

A method of controlled removal by evaporation of the aqueous solutionfrom dispersal agent coated SWCNT structures was conducted to observethe effect on the dispersal of the structures. Specifically, 25 μlsamples of an aqueous MβC solution, prepared according to the method ofExample 4, were deposited on 12 mm round glass coverslips. The aqueoussolutions were allowed to slowly evaporate by air drying over about a 12hour period. Subsequent AFM analysis of each coverslip revealed MβCcoated discrete SWCNT structures forming highly organized “rafts” or“tapes” as illustrated in a representative AFM image depicted in FIGS. 8a-8d (FIG. 8 a depicts the AFM height profile, FIG. 8 b depicts the AFMamplitude profile, and FIGS. 8 c and 8 d are magnifications of FIGS. 8 aand 8 b, respectively). The observed “raft” or “tape” SWCNT structuresextended hundreds of microns across the substrate and had various widthsranging up to 1 (μm but were no more than 6 nm in height. Additionally,it was observed that both single layers and multiple layers up to fourlayers thick of SWCNT structures had formed into highly orderedthree-dimensional geometries resembling a crystal structure. Thus, thedata confirms that controlled removal of the aqueous solution from thedispersal agent coated SWCNT structures results in the formation ofpurified and highly ordered, “raft-like” SWCNT structures rather thanropes or bundles of entwined nanotubes.

The present invention provides a significant improvement in theisolation and purification of individual and discrete SWCNT structuresby utilizing dispersal agents at effective concentrations tosubstantially disperse the structures in aqueous solution. Additionally,the present invention provides novel SWCNT structures that are easilycharacterized and useful for a variety of applications. Further,biological derivitization of the dispersal agent dispersions may beaccomplished with relative ease, thus leading to a variety of potentialapplications for SWCNT structures which may be easily manipulated.

The various technical and scientific terms used herein have meaningsthat are commonly understood by one of ordinary skill in the art towhich the present invention pertains. As is apparent from the foregoing,a wide range of suitable materials and/or methods known to those ofskill in the art can be utilized in carrying out the present invention;however, some preferred materials and/or methods have been described.Materials, substrates, and the like to which reference is made in theforegoing description and examples are obtainable from commercialsources, unless otherwise noted. Further, although the foregoinginvention has been described in detail by way of illustration andexample for purposes of clarity and understanding, these illustrationsare merely illustrative and not limiting of the scope of the invention.Other embodiments, changes and modifications, including those obvious topersons skilled in the art, will be within the scope of the followingclaims.

1. A medium for storing discrete carbon nanotubes comprising an aqueousdispersal agent solution containing nanotubes, with the nanotubesurfaces being coated with a dispersal agent, the dispersal agentpreventing the nanotubes from adhering to one another.
 2. The medium ofclaim 1, wherein the nanotubes includes single-walled carbon nanotubes.3. The medium of claim 1, wherein the dispersal agent is selected fromthe group consisting of detergents, surfactants, emulsifying agents,chaotropic salts, and ion pairing agents.
 4. The medium of claim 1,wherein the dispersal agent includes i synthetic detergent.
 5. Themedium of claim 1, wherein the dispersal agent includes anaturally-occurring detergent.
 6. The medium of claim 1, wherein thedispersal agent is selected from the group consisting of non-ionic,cationic, and anionic detergents.
 7. The medium of claim 1, wherein thedispersal agent is selected from the group consisting of alkylarylpolyether alcohols, phenylated polyethoxy ethanols, polyoxyethylenesorbitol esters, ammonium bromides and ammonium chlorides.
 8. The mediumof claim 1, wherein the emulsifying agent includes deoxycholates,taurocholic acid and salts thereof.
 9. The medium of claim 1, whereinthe emulsifying agent includes cyclodextrins.
 10. The medium of claim 9,wherein the cyclodextrins includes cyclodextrins having one or moresubstituted moieties.
 11. The medium of claim 9, wherein thecyclodextrins includes α-, β-, and γ-cyclodextrins.
 12. The medium ofclaim 11, wherein the cyclodextrins are selected from the groupconsisting of methyl-β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin.13. The medium of claim 1, wherein the dispersal agent includes asynthetic detergent in concentrations from about 50% to about 95% of itscritical micelle concentration value.
 14. The medium of claim 1, whereinthe dispersal agent includes a synthetic detergent having ahydrophilic-hydrophobic balance value from about 7 to about 13.2. 15.The medium of claim 3, wherein the emulsifying agent is present inconcentrations from about 5 mg/ml to about 500 mg/ml of aqueoussolution.
 16. The medium of claim 1, wherein the pH of the aqueousmedium is substantially neutral at ambient temperature.
 17. The mediumof claim 3, wherein the chaotropic salts include urea and guanidine. 18.The medium of claim 17, wherein the chaotropic salts is present inconcentrations from about 6M to about 9M in aqueous solution.
 19. Themedium of claim 3, wherein the ion pairing agents include sulfonicacids.
 20. The medium of claim 19, wherein the sulfonic acids include1-heptane-sulfonic acid and 1-octane-sulfonic acid.
 21. The medium ofclaim 3, wherein the ion pairing agents are present in concentrationsfrom about 1 mM to about 100 mM in aqueous solution.
 22. A devicecomprising assemblies of single wall carbon nanotubes arrayed on asubstrate, with the carbon nanotubes being coated with an agent thatassists in maintaining the nanotubes in a geometrically orderedconfiguration with respect to one another.
 23. The device of claim 22,wherein the nanotubes are assembled in a generally parallelconfiguration with respect to one another.
 24. The device of claim 22,wherein the nanotube assembly includes a monolayer.
 25. The device ofclaim 22, wherein the nanotube assembly includes a multilayer of atleast two layers of nanotubes.
 26. The device of claim 22, wherein thenanotubes include single-walled carbon nanotubes.
 27. The device ofclaim 22, wherein the agent includes a dispersal agent.
 28. The deviceof claim 23, wherein the dispersal agent is selected from the groupconsisting of detergents, surfactants, emulsifying agents, chaotropicsalts, and ion pairing agents.
 29. The device of claim 27, wherein thedispersal agent includes a synthetic detergent.
 30. The device of claim27, wherein the dispersal agent includes a naturally-occurringdetergent.
 31. The device of claim 27, wherein the dispersal agent isselected from the group consisting of non-ionic, cationic, and anionicdetergents.
 32. The device of claim 27, wherein the dispersal agent isselected from the group consisting of alkylaryl polyether alcohols,phenylated polyethoxy ethanols, polyoxyethylene sorbitol esters,ammonium bromides and ammonium chlorides.
 33. The device of claim 28,wherein the emulsifying agent includes deoxycholates, taurocholic acidand salts thereof.
 34. The device of claim 28, wherein the emulsifyingagent includes cyclodextrins.
 35. The device of claim 34, wherein thecyclodextrins includes cyclodextrins having one or more substitutedmoieties.
 36. The device of claim 34, wherein the cyclodextrins includesα-, β-, and γ-cyclodextrins.
 37. The device of claim 35, wherein thecyclodextrins are selected from the group consisting ofmethyl-β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin.
 38. The deviceof claim 27, wherein the dispersal agent is a synthetic detergent inconcentrations from about 50% to about 95% of its critical micelleconcentration value.
 39. The device of claim 27, wherein the dispersalagent is a synthetic detergent having a hydrophilic-hydrophobic balancevalue from about 7 to about 13.2.
 40. The device of claim 28, whereinthe emulsifying agent is present in concentrations from about 5 mg/ml toabout 500 mg/ml of aqueous solution.
 41. The device of claim 22, whereinthe pH of the aqueous medium is substantially neutral at ambienttemperature.
 42. The device of claim 28, wherein the chaotropic saltsinclude urea and guanidine.
 43. The device of claim 28, wherein thechaotropic salts are present in concentrations from about 6M to about 9Min aqueous solution.
 44. The device of claim 28, wherein the ion pairingagents include sulfonic acids.
 45. The device of claim 44, wherein thesulfonic acids include 1-heptane-sulfonic acid and 1-octane-sulfonicacid.
 46. The device of claim 28, wherein the ion pairing agents arepresent in concentrations from about 1 mM to about 100 mM in aqueoussolution.
 47. The device of claim 22, wherein the substrate includes apolymeric support film.
 48. The device of claim 22, wherein thesubstrate includes a surface-coated material.
 49. The device of claim22, wherein the substrate includes glass having a poly-hydroxylatedsurface coating.
 50. The device of claim 49, wherein the surface coatingincludes polyethylene glycol.